Air traffic service providers and regulators around the world are moving toward airspace and flight operations to enable greater flexibility and adaptability, along with assuring improved traffic flow, capacity, efficiency, and safety. A key part is the transition from radar surveillance to ADS-B (Automatic Dependent Surveillance-Broadcast) to track airplanes in flight and on the ground more accurately and reliably.

ADS-B is a new surveillance technology designed to help modernize the air transportation system. It provides foundational technology for improvements related to NextGen (Next Generation Air Transportation System) and SESAR [Single European Sky ATM (Air Traffic Management) Research Program]. NextGen refers to the effort of the U.S. FAA (Federal Aviation Administration) to transform the ATC (Air Traffic Control) system to support a larger volume of airplanes more efficiently. SESAR is a similar effort in Europe, the EU (European Community and Eurocontrol are the founding members of SESAR. 1)

For NextGen and SESAR, ADS-B is one of the most important underlying technologies in the plan to transform ATC from the current radar-based surveillance to satellite-based GPS (Global Positioning System) surveillance. In addition, the FAA states that ADS-B will serve as the cornerstone for this transformation, bringing the precision and reliability of satellite-based surveillance to the nation's skies.

Some background:

Air traffic surveillance as required in controlled airspaces nowadays predominantly uses ground stations equipped with PSR (Primary Surveillance Radar), SSR (Secondary Surveillance Radar) including Mode-S. Seamless and continuous flight surveillance as necessary in airspace with high traffic density and separation minima of 5, respectively 3 nautical miles, require an extensive ground infrastructure of radar stations, networks and surveillance data processing, as implemented in Central Europe, the U.S. or certain regions in Asia, thereby providing the necessary situation awareness to the controllers in the air traffic control centers. 2)

In the recent years ADS-B (Automatic Dependent Surveillance Broadcast) as a further surveillance technology has evolved. Modern Mode-S transponders on board of aircraft transmit the flight position and other information by so-called Extended Squitter messages (1090ES) on the 1090 MHz SSR-Mode-S downlink frequency (ADS-B Out). In the future radar systems will be complemented or even replaced by less costly ADS-B ground stations, which will be integrated in the existing surveillance infrastructure. The European ADS-B Implementing Rule requires that new aircraft heavier than 5700 kg or faster than 250 knots will be equipped with ADS-B-Out from 2015 onwards when flying IFR (Instrument Flight Rules), and for already operational aircraft a retrofit from end of 2017 on. In 2020 ADS-B surveillance shall become operational. 3)

Most regions of the world are uncontrolled airspace. In areas without radar coverage, referred to as NRA (Non Radar Airspace), like oceanic airspaces, polar regions or structurally lagging continental regions the installation of ground stations is either impossible or too expensive. Today, aircraft surveillance in these regions is applied procedurally, i.e. by voice radio position reports of the pilots when the aircraft reaches certain waypoints. Also ADS-C (Automatic Dependent Surveillance – Contract) is used, a point-to-point data link connection (FANS1/A / Satcom), which transmits positional and other flight information only every 15 minutes due to limited bandwidth. In both cases no seamless and continuous flight surveillance is possible, with the consequence of relatively ample separation distances due to safety reasons. This becomes especially problematic for search and rescue activities in case of flight accidents: the location of the impact site of the crashed AF447 flight from Rio de Janeiro to Paris in 2009 took more than five days. However, it must be stated with regard to a recent fatal accident, that either a technical failure of the transponder or the navigation system, from where the transponder gets the actual aircraft position, or a manual deactivation will prevent an aircraft from being tracked via its ADS-B signals.

In 2008, the German Aerospace Center (DLR) started to investigate the option to receive the 1090ES ADS-B signals broadcasted by aircraft on board of LEO (Low Earth Orbiting) satellites. The efforts resulted in the DLR project ADS-B over Satellite (AOS), with the goal to develop an ADS-B payload for an IOD (In-Orbit Demonstration) and thereby demonstrate the feasibility of worldwide satellite based ADS-B surveillance.

This AOS In-Orbit Demonstrator is capable to receive, decode and forward all Mode-S downlink telegram formats. This includes the DF17 Extended Squitter comprising ADS-B information and DF11 All-Call reply. The AOS IOD was conducted in the frame of ESA's PROBA-V mission (PROBA-Vegetation) and was successfully launched on top of Europe's newest launch vehicle VEGA on May 7, 2013 from Kourou in French Guyana. 4)

The IOD is a first step for demonstration and verification of space based air traffic surveillance. A single satellite is equipped with a space-qualified ADS-B receiver, and due to limitations in cost, time and in particular resources available on PROBA-V in terms of power and geometry a relatively simple antenna and receiver design was implemented. A future operational system providing seamless world-wide coverage would consist of a fleet of satellites, each equipped with sophisticated multi-channel ADS-B receivers and antennas.

ADS-B over Satellite is the first experiment of its kind and has already proven the feasibility of space based ADS-B. The results from this IOD will pave the way for future developments towards global satellite based air traffic surveillance.

Terrestrial ADS-B:

ADS-B is considered as an essential component of any future air traffic management system. It is incorporated in the U.S. NextGen (Next-Generation Air Transportation System) as well as in the SESAR [Single European Sky ATM (Air Traffic Management) Research] initiative and will provide enhanced surveillance capabilities. Ground based ADS-B stations are increasingly deployed, but the coverage area is limited typically to a few hundreds of kilometers. Air Services Australia, as an example, installed a huge number of ADS-B ground stations in order to cover the entire continent above FL (Flight Level ) 300 eventually.

However, an adequate solution for a global surveillance of air traffic movements based on ground based ADS-B appears to be out of scope due to technical, operational and political constraints:

- Oceanic Coverage would implicate the deployment of ADS-B stations on innumerous buoys.

- Terrestrial Coverage would implicate the deployment and operation of ADS-B stations in inaccessible terrain.

- The global airspace is fragmented and thus operated by a large number of local ATC providers.

- Political obstacles in particular in unstable regions prevent any transnational regulation and operation.

The 1090 MHz Mode-S Extended Squitter (1090ES):

SSR including Mode-S is using an uplink frequency of 1030MHz in order to interrogate aircraft within operating coverage. Possible interrogations are e.g. the Mode-S-All-Call (any aircraft within the coverage will respond to the call) or selective interrogations for identity, altitude and other information using the worldwide unique technical Mode-S-address assigned to every aircraft. Once an aircraft has been interrogated, it will reply on the 1090MHz downlink channel. As ADS-B is an automatic broadcast system, it does not need an interrogation and just makes use of the 1090 MHz Mode-S downlink format DF17 employing a PPM (Pulse Position Modulation) and random channel access. ADS-B messages are generated at intervals specified for the diverse message types as given in the 1090 MHz Extended Squitter minimum operational performance standards, and transmitted alternately from the top and the bottom ATC antenna of an aircraft. The DF17 contains position, altitude, identity, flight direction, speed and aircraft status in consecutive messages. At the receivers side the messages will be assembled to ADS-B reports, using the Mode-S address comprised in the messages.

Spaceborne ADS-B:

Only satellites have the capability to provide a global coverage at any possible flight level, avoiding limitations imposed by terrestrial ADS-B. This could be implemented by receiving ADS-B signals, which are broadcasted regularly by each equipped aircraft and which contain information on position, speed, direction etc. by LEO satellites. This data can then be made available to already existing ATC ground infrastructures.

Therefore, a satellite-based surveillance network will provide enhanced Air Traffic Services in areas where the traffic density, the location, or the cost of "conventional" ATC equipment would not justify any installation of radar and/or terrestrial ADS-B. It can also include VHF coverage fringe areas and areas where existing radar is to be de-commissioned, and where the replacement costs are not justified.

First ADS-B receiver demonstration on the PROBA-V minisatellite

The focus of "ADS-B over Satellite" is primarily on the en-route phase of aircraft. Departure/climbing and/or approach/landing as well as the Taxi inbound and outbound TMA (Traffic Management Advisor) areas, was not concentrated upon.

The primary goal of the DLR project ADS-B over Satellite was to demonstrate the feasibility of an orbital ADS-B system by means of an In Orbit Demonstration and to evaluate the characteristics and performances which may be important for future space based air traffic control systems.

In initial experiments, the project has already proved to be successful. In 2009, during a series of high-altitude balloon flights in northern Sweden, the receiver was able to pinpoint an aircraft flying 1100 km away, from a height of about 30 km. For example, the project could 'see' a flight from Beijing to Amsterdam over the North Sea. In a further experiment in 2012, the researchers flew their receiver on a balloon at an altitude of 40 km and examined the interfering signals that it must cope with in a heavily flown and radar-monitored area. During these test flights, a terrestrial ADS-B receiver has been used and basic assumptions regarding the maximum reception distance in NRA could be verified. 5)

Based on these trials and pre-development studies a system concept for a spaceborne ADS-B reception has been developed, mostly based on COTS (Commercial of-the-Shelf) hardware. This system is based on two assemblies consisting of a passive planar L-band antenna array and the associated receiver unit.

The idea with this ADS-B receiver system is to take it as a hosted payload on a satellite, foregoing any costly equipment upgrades, and investigate if it is technically feasible to receive ADS-B signals in orbit. PROBA-V will demonstrate how many aircraft can be observed worldwide and which types – different-sized aircraft are assigned ADS-B systems with differing signal strengths. Over the next two years, researchers intend to test, for the first time, whether continuous monitoring of aviation routes is possible. At present, this cannot be achieved in non-radar airspace; location monitoring from space could close this gap.

When an aircraft flies over the major oceans, large areas without infrastructure or the Polar Regions, it is no longer trackable by ground radar stations – the range of the stations is insufficient. But the aircraft continuously transmit ADS-B signals, with information such as altitude and speed — the DLR project team wants to make use of this.

Launch: The PROBA-V spacecraft (mass of 140 kg) of ESA was launched on May 7, 2013. The launch vehicle was Vega (with Arianespace as launch provider); the launch site was the Guiana Space Center, Kourou. This marks the first VERTA (Vega Research, Technology and Accompaniment)flight of VEGA (designated as VERTA-1). The mission is designated Flight VV02 in Arianespace's numbering system. 6)

Orbit of PROBA-V: Sun-synchronous orbit, altitude = 820 km, inclination = 98.8º, LTDN (Local Time on Descending Node) = 10:30 hours (with a drift limited between 10:30 and 11:30 AM during the mission lifetime). Note: In contrast to the SPOT-4 and SPOT-5 missions, PROBA-V will not have the capability to maintain its orbit.

The ADS-B receiver on PROBA-V is provided by DLR and SES Techcom of Luxembourg, the main objective is to test (space qualify) the ADS-B electronic boards in flight-representative configuration to evaluate TID (Total Ionizing Dose). The basic design concept of the ADS-B receiver (1090ES RX) is a single conversion superheterodyne receiver with a down conversion of 1090 MHz to an intermediate frequency of 70 MHz. The IF sampling at 70 MHz is done by a 16 bit ADC at 105 Msps (Mega samples per second).

The digital part of the receiver is built around a Cyclone IV FPGA from Altera which combines the complete data processing as well as the communication with the onboard computer of the PROBA-V spacecraft. The digital and the RF section of the receiver are built on an individual PCB each, connected with a 37 pin MDM PCB connector. 7)

The FPGA comes with a 32 bit embedded processor to handle the satellite-borne interface. The communication to the spacecraft and commanding of the receiver is done through RS-422 UARTs (Universal Asynchronous Receiver/Transmitters) at a baud rate of 115 kbit/s. The input sensitivity of the receiver depends on the frequency condition of the incoming message and has a minimum trigger level of about -96 dBm.

Spaceborne ADS-B antenna: The antenna used for ADS-B over Satellite is an antenna array of two elements. Each element is a capacitive fed, shorted patch antenna. There is no direct mechanical bonding between the feeding structure and the patch. This makes the patch assembly very easy. Except for the feeder no dielectric material is used. It has been found that this gives an extra 12% gain increase. The patch antenna is shorted in the center of the patch to the ground plane of the spacecraft structure. This avoids potential charging.

Antenna type

Planar with two elements

Frequency

1090 MHz

Gain

11.2 dBi

HPBWAZ (Half Power Beam Width) in azimuth

33º

HPBWEL

73º

Polarization

Right Hand Circular

Table 1: Simulated antenna characteristics

For the project team, tracking flights from a satellite is new territory. So far, no satellite has been used to receive ADS–B signals. In this first test, the characteristics of how aircraft radiate the ADS–B signal will be recorded. In the frame of the project, SES Techcom developed and implemented the ground data processing center, which retrieves, processes, analyses and stores all ADS-B data received from the PROBA-V satellite. 8)

The pioneering ADS-B payload will be followed by the in orbit validation mission, which will demonstrate the full technical scope of spaceborne ADS-B. ESA (European Space Agency) has contracted Thales Germany for the development of this next generation ADS-B system, which is progressing on schedule with strong participation of the Luxembourg space industry, such as LuxSpace, with the TRITON microsatellite platform to support the future demonstration mission.

Status of ADS-B on PROBA-V:

• May/June 2016: The world's first ADS-B over Satellite (AOS) In-Orbit Demonstrator (IOD) within ESA PROBA-V mission is operational since May 2013 and has successfully validated the principle of detecting weak Mode-S transponder transmissions from a LEO (Low Earth Orbit). A special feature was included in the receiver's firmware that allows to upload new configurations and to activate these by remote access. During mission runtime so far, this has been successfully tested several times. In the meantime, an improved Mode-S correlation mechanism was developed that benefits from the phase coherence of the pulse train from the first Mode-S preamble pulse to the fifth format bit. In lab tests it could be shown that the telegram detection rate increased significantly. Moreover, by generating and saving "Low-Confidence Bits" for the 112 Mode-S data bits in DF17, there is an additional chance to increase the success rate for error-free demodulation of the telegram in postprocessing. 9)

• The ADS-B receiver on board the satellite was the first experiment of its kind, receiving 1090ES ADS-B squitter signals transmitted from aircraft. Therefore the experimenter could not build on experiences or any evaluation results.

• The assessment of the achieved results should take into account the constraints under which this experiment was realized, as there were limitations in cost and time but in particular available resources on the satellite in terms of available power and geometry.

• The reception of 1090 Extended Squitter ADS-B messages on board of the PROBA-V satellite is mainly affected by the following issues, which may lead to a loss of ADS-B information:

- RF signal loss due to the low signal level resulting from the distance between the receiving satellite at an altitude of approximately 820 km and the transmitting aircraft at an altitude of 0 to 12 km.

- RF signal loss due to the shapes of the satellite antenna vertical radiation pattern and the aircraft antenna vertical radiation pattern.

- Corruption of messages by garbling, when several messages arriving at the ADS-B antenna onboard of the satellite at the same time overlap and thus cannot be decoded by the ADS-B receiver.

- Speed of the satellite of about 27000 km/h, which leads to a limited time of observation for each detected aircraft of about 3 minutes maximum (Ref. 2).

Figure 3 shows aircraft tracks recorded world-wide during pass 2699 on 11th February 2014. A the pass covers several orbits around the Earth between two consecutive acquisitions of PROBA-V from the satellite operation center Redu in Belgium. Each red dot represents the track segment of an aircraft as seen by the satellite when its orbit passes the aircraft. With a size of the antenna footprint of approximately 1200 km in longitudinal and 500 km in lateral extension to the satellite's flight direction, PROBA-V scans the global airspace strip by strip.

Antenna footprint: The most important aspect for the reception of ADS-B messages in space are the receiving conditions for the 1090 MHz extended squitter signal on board the satellite. In comparison to ground based ADS-B surveillance with a range of up to 300 km maximum, the signal path between a LEO satellite orbiting at 820 km altitude and an aircraft is much longer, which results in a low signal level at the ADS-B receiver. Thus, the Mode-S signals have to be detected nearly at noise level by a correlation process.

The shape and extent of the footprint was determined by compiling histograms, which reflect the number of received messages with respect to the aircraft position in relation to the satellite position projected to an x-y plane. Figure 4 shows a histogram for all decoded position messages received during May 2014.

Remarkable are the two peaks, one lower in front of the movement direction and the other higher behind the satellite, with a dip at the nadir position. The footprint is not symmetrical, which is caused by the asymmetric mounting positions of the patch antennas on board of the satellite, as well as other equipment mounted at the underside and a solar panel which protrudes over the front edge of the lower surface.

Figure 4: Histogram of all received position messages in the antenna footprint (image credit: DLR)

The peaks of the histogram can be explained by the antenna radiation patterns of the receiving antennas of the satellite and the transmitting antenna of the aircraft. Figure 5 shows the measured radiation pattern of the ADS-B patch antennas mounted on the nadir panel of PROBA-V. The resulting combined antenna radiation pattern has an overall oval shape in the direction of the satellite movement and the maximum sensitivity is slightly behind the nadir direction below the satellite.

In summary, the results gained so far by the PROBA-V in-orbit demonstration prove, that space based 1090ES ADS-B surveillance is technically feasible, and thus the goals of the project "ADS-B over Satellite" have been attained (Ref. 2).

• June 13, 2013: An A320 Airbus overflying Scotland was the first aircraft 'seen' from space by a new receiver, the ADS-B device of DLR. This verifies that tracking aircraft from space is possible. 10)11)

• On May 23, 2013, the ADS-B experiment was switched on for the first time, recording over 12,000 ADS-B messages within two hours, at an altitude of 820 km. The project team detected over 100 aircraft during the first pass over the British Isles, East Asia and Australia when the receiver was switched on.

• May 17, 2013: The PROBA-V spacecraft is in good health following its launch on May 7, 2013.12)

Figure 6: Tracking aircraft with the ADS-B receiver on board PROBA-V over Great Britain and the Atlantic Ocean (image credit: DLR)

Launch: Iridium has contracted with SpaceX to launch the Iridium NEXT constellation on Falcon-9v1.1 vehicles in the timeframe 2015-2017. Ten satellites are on each launch and seven missions are planned.

• ITT-Exelis, provider of systems engineering support, under contract to Aireon and builder of processing and distribution subsystem.

• NAV CANADA ‐ Investor in Aireon; launch customer for ADS‐B service.

Background: In November 2012, Iridium Communications Inc. announced that it had finalized an agreement with NAV CANADA regarding Aireon LLC, a joint venture that will allow air traffic management agencies around the globe to continuously track aircraft anywhere in the world. For the first time ever, ANSPs (Air Navigation Service Providers) around the world will be able to track aircraft from pole-to-pole, including oceanic airspace and remote regions. The new capability will provide significant benefits to the aviation industry, including substantial fuel savings, a reduction in greenhouse gas emissions and enhanced safety and efficiency for passengers. The venture will be operated under a PPP (Public Private Partnership) between the U.S. FAA (Federal Aviation Administration), industry and the world's major ANSPs. 13)14)

The formation of Aireon LLC, a joint venture with Iridium Satellite LLC, (a subsidiary of Iridium Communications Inc.) was announced on June 19, 2012 at a news conference in Washington D.C. The agreement finalizing the terms of NAV CANADA's participation in Aireon was signed in November 2012. NAV CANADA is a commercial corporation that owns and operates Canada's civil ANS (Air Navigation Service).NAV CANADA is ideally suited to be the first customer of Aireon, because it manages the second largest air navigation service and is the largest provider of oceanic services in the world by traffic volume. 15)16)

In March 2014, Aireon LLC announced that the ADS-B receiver payload for the system has successfully completed qualification testing for operation in the harsh environment of space. This achievement is a key building block in the deployment of Aireon and represents a significant milestone in validating that the payload and system design will provide robust reception of ADS-B signals from space. The company will now begin production of 81 ADS-B 1090 ES (Extended Squitter ) receiver payloads, designed by Harris Corporation, that will be hosted on the Iridium NEXT satellite constellation, with launches to start in 2015. The receiver payloads will complement ground-based air traffic surveillance systems currently in use by seamlessly relaying position and status information of aircraft flying over oceans, poles and remote regions to air traffic controllers on the ground. 20)

The U.S. FAA (Federal Aviation Administration) announced in May 2010 the performance requirements for aircraft tracking equipment that will be required under NextGen (Next Generation Air Transportation System. The avionics will allow aircraft to be controlled and monitored with greater precision and accuracy by a satellite-based system called ADS-B (Automatic Dependent Surveillance – Broadcast). 21)

The final rule, developed with extensive input from the aviation community, requires aircraft flying in certain airspace to broadcast their position via ADS-B by January 1, 2020. The rule mandates that the broadcast signal meet specific requirements in terms of accuracy, integrity, power and latency.

NextGen is the transformation of the radar-based air traffic control system of today to a satellite-based system of the future. This transformation is essential in order to safely accommodate the number of people who fly in the United States. NextGen represents an evolution from a ground-based radar system of air traffic control to a satellite-based system of digital standards for air traffic management. More significant, however, is the movement away from disconnected and incompatible information systems to a scalable network-centric architecture in which everyone has easy access to the same information at the same time.

Present (convential) system

NextGen system

Ground-based navigation and surveillance

Satellite-based navigation and surveillance

Voice communications

Digital communications

Disconnected information systems

Networked information systems

Disparate, fragmented weather forecast delivery system

Single, authoritative system in which forecasts are embedded into decisions

Under the FAA's new rules, only the ADS-B "Out" transmission capability is required. There is no mandate for ADS-B "In." However, this optional "In" capability — which receives the tracking data for display in the cockpit — should be a popular upgrade, since it can clearly enhance situational awareness by giving pilots a view of the same basic traffic data that ground controllers are monitoring on their scopes. Additional FAA inducements for adding ADS-B "In" include free datalink weather and various other flight information services. 22)

The FAA has designated two options for airborne equipment that will satisfy the ADS-B "Out" requirement:

• One is the dedicated 978 MHz UAT (Universal Access Transceiver)

• The other option is the pairing of a 1090 MHz Mode-S "extended squitter" transponder with an approved GPS navigation source (such as WAAS GPS) to provide the required position, vector, altitude and velocity data.

The FAA has already begun to implement the nationwide infrastructure of ground stations needed to support ADS-B. By 2013, the FAA expects to have some 790 ground stations, linked through 4 data-control centers, providing full ADS-B coverage for more than 95 percent of the airspace over the contiguous United States.

When fully implemented, NextGen will safely allow more aircraft to fly closer together on more direct routes, reducing delays and providing unprecedented benefits for the environment and the economy through the reduction of carbon emissions, fuel consumption and noise.

The information compiled and edited in this article was provided byHerbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).